The invention relates to the determination of a quantitative statement concerning the quality of a measurement signal, preferably a medical measurement signal, such as in pulsoximetry.
Measuring of signals usually comprises a process in several stages, typically with the steps of signal recording, signal processing, and signal evaluation. In the simplest case of a measurement, the mere signal recording suffices, but depending on the application the signal processing and/or evaluation are also regularly necessary.
In signal recording, signals representing the quantity to be measured are recorded as basic signal values, for example by means of a sensor or some other suitable recording device. In some cases, for example in electrocardiography (ECG), these recorded basic signal values already directly represent the measurement quantity to be determined (also called parameter). In other cases, for example the determination of oxygen saturation (SpO2), the recorded basic signals indirectly represent the desired measurement quantity, and a signal evaluation is still necessary in each case so as to derive the desired measurement quantity from the basic signals.
Depending on the measurement process, the measurement accuracy, and influences on the measurement, it is necessary for the basic signals (both those directly and those indirectly representing the measurement quantities) to undergo a signal processing, i.e. the basic signals must be suitably adapted, for example through an improvement in the signal quality such as the signal to noise ratio, or through filtering or suppression of undesirable measurement influences.
Similarly, a signal evaluation is also necessary, depending on the measurement process and the measurement quantity, so as to obtain from the recorded basic signals or the processed signals the desired measurement value of the measurement quantity. As was noted above, the quantities representing the measurement quantity indirectly only are to be evaluated, because the obtained basic signals by themselves are not conclusive.
Given the fact that the obtained measurement values represent the result of the measurement process, or that further quantities, conclusions, or consequences are derived from these measurement values, the question often arises in how far the obtained measurement values can be relied on, i.e. how well or how badly these measurement values represent the actual values of the quantity measured. The reliability or quality of the measurement values is of major importance in particular in the field of medical applications, such as patient monitoring, because a measurement value incorrectly representing the quantity to be measured, while this incorrectness is not observable, may have serious consequences for a patient""s life and health, for example owing to an incorrectly prescribed or omitted therapy, or owing to a suppressed alarm function.
A good example of the importance of the quality of measurement values is found in pulsoximetry, where this has frequently led to problems in the past, while at the same time the requirements were set higher and higher. Pulsoximetry is a non-invasive, continuous determination of the oxygen content of the blood (oximetry) based on an analysis of the photospectrometrically measured pulse. It is necessary in this field that a pulse curve (plethysmogram) should be available at several wavelengths. Practically all appliances operate at only two wavelengths, which renders possible inexpensive, compact solutions. The principle of photometry is based on the fact that the quantity of absorbed light is determined by the degree of absorption of a substance and by the wavelength. Pulsoximeters utilize this in that the arterial blood volume, and only the arterial blood volume, pulsates in the rhythm of the heartbeat. The basic principle and the application possibilities of pulsoximetry are generally known and have frequently been described, in particular in EP-A-262778 (with a good summary of the theory), U.S. Pat. No. 4,167,331, or Kxc3xa4stle et al. in xe2x80x9cA New Family of Sensors for Pulsoximetryxe2x80x9d, Hewlett-Packard Journal, vol. 48, no. 1, pp. 39-53, February 1997.
Although ever more difficult cases, as regards the signal quality of the pulsoximetric measurement, are still used for deriving measurement values, there has until now not been a conclusive indicator for the clinical user which renders it possible to evaluate conclusively the reliability and quality of the measurement values obtained. Such an evaluation, however, is important because the pulsoximeters with their plethysmographic basic signals contain too little information for deciding with full certainty in boundary cases whether a measurement value can be indicated. A doctor, for example, regularly has substantially more information available to him for deciding whether the oxygen supply of his patient is actually critical or whether there is merely an artifact of the pulsoximeter.
The aids frequently available to the user of the pulsoximetrical measurement are a representation of the basic signal in the form of a curve (plethysmogram) or the one-dimensional representation as a pulsating histogram bar. In addition, there are warning signals such as xe2x80x9cmotionxe2x80x9d, xe2x80x9cnoisexe2x80x9d, xe2x80x9clow signalxe2x80x9d in text fields on displays, or flashing displays (cf. Maurice et al., A Comparison of Fifteen Pulsoximeters, Anesth. Intens. Care, vol. 17, pp. 62-82, 1989).
A first step towards achieving a quality indicator for the pulsoximetrical measurement values can be found in EP-A-587009. A bar display is described therein which is controlled by the transmission (light transmission of the tissue), i.e. by the absolute signal strength (DC component) of the measurement signal. This system, indeed, is certainly capable of indicating an unsuitable measuring location, for example because the received luminous intensity is too low. Nevertheless, the AC component (perfusion) and the interference level remain unconsidered, so that an indication with a high rating for the quality level does not give any guarantee for a reliable measurement.
The user may indeed get a certain indication as to the quality of the signal through judging the size and shape of the plethysmogram and the other displayed quantities. The conclusiveness thereof, however, is limited. A problem is, for example, that only one basic signal is often displayed as the plethysmogram, while the pulsoximeter evaluates two basic curves.
The N-400 Fetal Oxygen Saturation Monitor from the Nellcor Puritan Benett Company has at its front a triangular rod display serving as a multi-parameter reliability indicator for the average signal quality. This signal quality indicator represents the quality of the signal which is used for calculating the SpO2 value. When the signal quality drops to below a required threshold value, an acoustic alarm signal is triggered upon the loss of the signal.
A further method of testing the obtained pulsoximetric measurement value is described in EP-A-904727 and is based on a comparison of the pulse frequency of the pulsoximeter with the heartbeat frequency of the ECG signal. If, for example, the deviation between the two lies within a range of only a few beats per minute, the pulsoximeter is considered credible. If this difference is greater, however, the pulsoximeter readings are considered doubtful.
It is a disadvantage in most of the methods of determining the credibility of the pulsoximetrical measurement value mentioned above that at most only half of the measurement signals available is actually used. Thus, for example, only one of the basic signals suffices for deriving the pulse rate, so that the function of the other channel usually remains fully untested. Alternatively, the periodicity of the signal only is evaluated, while, for example, the amplitude of the basic curve remains substantially unconsidered. This results in numerous potential errors, which are often not even noticeable. If erroneous measurement values or erroneous quality indicators for the measurement values lead to, for example, alarm functions being triggered, and such an erroneous triggering of the alarm is recognizable to the clinical personnel, said alarm functions are often switched off in the clinical operation so as to avoid unnecessary alarms. This, however, may lead to the possibly fatal consequence that the switching-off also means that actual alarm situations are not recognized or are recognized too late. Conversely, an error which is not recognized or heeded may have the result that the obtained measurement value is trusted, although it deviates from the actual value. Life-threatening situations may thus remain unrecognized owing to the reliance placed on the correctness of the measurement value.
It is accordingly an object of the present invention to provide an improved statement on the reliability of obtained measurement values, in particular in medical applications such as pulsoximetry. The object is achieved by means of the characteristics of the independent claims. Advantageous embodiments are defined in the dependent claims.
According to the invention, the determination of a quantitative statement concerning the quality of a measurement signal, preferably a medical measurement signal such as in pulsoximetry, takes place through the determination of factors which preferably relate to signal recording, signal processing, and/or signal evaluation. A link is established between these determined factors by combinatory processes, in particular by an uncertain logic such as, for example, fuzzy logic, so as to obtain a quality indicator which quantitatively describes the quality of the determined measurement value.
Available examples of factors relevant to signal recording are factors which describe the measurement location, the measurement time, the measurement sensor system, or the like.
Factors relevant to signal processing may be, for example, the signal-to-noise ratio, parameters of a possible subsequent noise suppression, or signal compression, or the like.
Factors relevant to signal evaluation may be determined in particular by the measurement algorithm(s) used, consideration of all basic signals or only part thereof.
It is obvious that the inclusion of as many as possible of such relevant factors, depending on the application, renders it possible to improve the xe2x80x9cqualityxe2x80x9d of the quality indicator. It is alternatively possible, however, to restrict the links to only a few selected factors, in dependence on the application or subject to, for example, computer limitations.
The quality indicator is preferably visually represented by a suitable display, for example between a minimum value and a maximum value. The maximum value is shown in the case of an ideal signal, indicating the highest degree of reliability. As the noise component increases, the quality indicator will drop down to the lower limit of the minimum value, indicating the lowest degree of reliability. In that case the signal is so weak or so strongly noise-laden that it is highly probable that the derived measurement values have major errors and should accordingly not be displayed anymore, or only with a suitable warning.
The level of the quality indicator thus provides a relative measure as to the reliability which the measurement values may be presumed to have. It is clear, however, that an exact prognosis of reading errors cannot be guaranteed, especially not in each and every individual case. Lower values of the quality indicator (low reliability), however, do make the user aware that the readings may be doubtful. The user may then test them by alternative means or may try to achieve a better signal quality, and thus a more reliable measurement, for example through the choice of an alternative measurement location or the use of a different sensor. The quality indicator according to the invention supplies the relevant clues for this.
The relevant factors are preferably linked through the use of known principles of fuzzy logic as described in particular in Altrock C. xe2x80x9cFuzzy Logic: Band 1. Technologiexe2x80x9d (Fuzzy Logic: Part 1. Technology), Oldenburg Verlag, Munich, 1995, so that these principles need not be discussed here in any detail and a reference to this and other background literature suffices. Reference is also made to the standard work on statistics by Kreyszig E. xe2x80x9cStatistische Methoden und ihre Verwendungxe2x80x9d (Statistical Methods and Their Use), Vandenhoek and Ruprecht, Gxc3x6ttingen, 1975, with regard to statistical criteria and algorithms.
In a preferred embodiment, the quality indicator according to the invention is presented in the form of a tendency indication in the sense of xe2x80x9crelatively better/worsexe2x80x9d or xe2x80x9cabsolutely good/badxe2x80x9d, for example by means of a quasi-analog display with histogram bars of variable length. This renders possible a satisfactory and sufficiently intuitively recognizable representation of the quality indicator according to the invention.
In another preferred embodiment, a further visualization of the quality indicator according to the invention takes place such that the display method for the measurement quantity described by the quality indicator changes when the quality indicator value exceeds one or several given threshold values. This may take place, for example, through a switch-over from a continuous display to a flashing display (possibly with a varying flashing frequency in dependence on the quality indicator value), through a change in the display color (for example, red in the case of low quality indicator values or as in a traffic light: green for a high, amber for an average, and red for a low quality indicator value), or by an inversion of the colors. This not only gives an intuitively recognizable impression of the display value of the quality indicator but also a reliable call to attention in the case of any critical measurement situations.
In a further preferred embodiment of the invention, an alarm function is controlled (for example upon a deviation of the measurement signal from a given value or range) in dependence on the quality indicator. Preferably, such a control takes place through a change in an alarm delay period (i.e. the time between the moment an alarm-triggering criterion is reached and the actual triggering of the alarm) in dependence on the value of the quality indicator. Since the quality indicator is a measure for the signal quality, the alarm delay time is preferably set for a maximum in the case of a bad signal and an accompanying low quality indicator value (=low reliability), because the probability of a false alarm is very high here. At high values of the quality indicator (=high reliability), the risk of a false alarm is small, and the alarm delay time may be reduced, which in its turn provides a chance of a fast recognizability in the case of actual alarm situations.
In a further embodiment, an evaluation of the gradient in time of the quality indicator takes place so as to be able to recognize trends and to derive therefrom, for example, error prognoses. Preferably, such a trend is displayed in addition to the display of the quality indicator value, for example through a display of an arrow pointing upwards for an improvement of the signal quality, an arrow pointing downwards for a deterioration of the signal quality, and an arrow remaining at the same level for indicating a signal quality which remains substantially the same. Furthermore, an error prognosis is possible by means of the gradient of the quality indicator trend, so that an alarm signal can be given, for example in the case of a continuous downward trend of the quality indicator value over a given period, also if the absolute value of the quality indicator should still be within an acceptable range.
The values of the determined quality indicator and/or the trend of the quality indicator are preferably recorded at the same time so that they can be taken into account in a subsequent evaluation of the measurement. This renders it possible, for example, to qualify measured occurrences as artifacts or as genuine occurrences later.
The invention is used preferably for medical measurements and monitoring, for example in pulsoximetry, but it is not limited thereto and may also be used for alternative applications.